1. Field of the Invention
The present invention relates to a method and apparatus for recording a hologram, a method and apparatus for reproducing the hologram, and an information retaining body, and particularly to a method and apparatus for recording a hologram, a method and apparatus for reproducing a hologram, and an information retaining body, in which information recorded as a hologram in a holographic recording layer is reproduced from the holographic recording layer added onto a support body, particularly onto a sheet member having a non-optical quality such as paper or a plastic card.
2. Description of the Related Art
Recently, with development in information communication equipment, development of infrastructure, and wide spread of personal computers, information can be processed on a computer, while many pieces of information can be obtained through electronic media which record and transmit information formed in an electronic format. On the other hand, paper media have been used as information media which record and transmit information. To this day, transmissions and exchanges of various pieces of information are performed by paper media such as post cards, letters, books, and newspapers.
Both the electronic media and the paper media have respective advantages. The electronic media are superior in processing properties, reusability, searching properties of the information and ease of correlation. On the other hand, the paper media are superior in at-a-glance properties, portability, and realism. Therefore, instead of the paper media being replaced with the electronic media, it is ideal that the paper media and the electronic media are complementary to each other while utilizing the advantages of both.
Information on the electronic media can be converted into information on the paper media by “printing.” However, digital information of the electronic media may be lost during a printing process. Further, information on the electronic media can be converted into information on the paper media by a character recognition apparatus such as an OCR (Optical Character Reader). However, in the technique for recognizing an image, there are limits with respect to recognition performance and conversion speed.
In order to solve these problems, various methods for printing a code, by which digital conversion can be performed, such as a glyph code or a two-dimensional barcode in the paper media have been proposed. For example, a system which merges information on the WWW (World Wide Web) and information on paper media by expressing URL (Uniform Resource Locators) concerning the information printed on the paper media with a two-dimensional code and printing the two-dimensional code on the paper media (refer to “A Study for linking between WWW and paper by 2D code”, Human Interface 76-1, Information Media 33-1(1998)).
For example, a “QR code” (trademark) is available for the two-dimensional code. 134 bytes of data can be stored by utilizing the QR code (trademark) having 41 by 41 cells, and this is sufficient to describe URL information or ID information. The QR code (trademark) imparted to the paper media can be read by using the system to access the URL on the WWW, and the electronic information corresponding to the information on the paper media can be obtained as necessary.
Recently, a “μ-chip” (trademark) by which stored data can be read in a non-contact manner and which can be embedded in the paper and the like has been developed (refer to MYCOM PC WEB, NEWS HEADLINE, Jul. 5, 2001; http://pcweb.mycom.co.jp/news/2001/07/05/22.html). The information can be read in the non-contact manner by embedding the μ-chip (trademark), in which the URL information or the ID information is stored, into the paper media.
A printing apparatus that uses a transfer foil ribbon in which a holographic pattern is formed in advance to transfer and print a holographic image into a display medium such as paper with a thermal head, and a display medium produced by the printing apparatus have been proposed (Japanese Patent Application Laid-Open No. 2000-211257).
However, the above-described two-dimensional code and semiconductor chip only have a storage capacity as small as several hundred bytes. Although this storage capacity is enough for the URL information or the ID information, the storage capacity is not sufficient for the utilization of various kinds of information.
Because a thickness of the μ-chip (trademark) ranges from about 60 μm to about 120 μm, the μ-chip (trademark) can be embedded only into so-called thick paper having a thickness not lower than 200 μm. Further, the information is transmitted by wireless communication, so that a mechanism on the information retaining media side and an encryption technique tend to be complicated.
The hologram has a larger storage capacity compared with the two-dimensional code or the semiconductor chip, the holographic recording layer can be formed in a sheet, and the holographic recording layer can be easily bonded to the paper media. However, because the hologram, in which a signal light beam (object light beam) is recorded, is irradiated with a reference light beam to reproduce the signal light beam by diffraction from the hologram, there is a problem in that the signal light beam cannot be read with a high S/N ratio due to diffuse reflection on a surface of the sheet member or flexure of the sheet member, when the holographic recording layer is bonded onto a sheet member having a non-optical quality such as paper or a plastic card.
In the case of a transmission type hologram in which the recording is performed by irradiating the holographic recording layer from both the same side with the signal light beam and the reference light beam, at an ongoing side of the signal light beam, a thin film reflection layer is bonded to the sheet member. However, the surface of the reflection layer is formed along irregularity of the surface of the sheet member, so that the reproduced signal light beam is scattered when reflected by the reflection layer during holographic reproduction. As a result, reading errors are increased. In the case of a reflection type hologram, in which the recording is performed by irradiating the holographic recording layer from different sides with the signal light beam and the reference light beam, the reproduced signal light beam is not affected by the irregularity of the surface of the sheet member. However, the reference light beam is scattered on the surface of the sheet member, and the reproduced signal light beam is submerged in the scattered light beam, so that the reproduced signal light beam can not be detected.
In order to solve the problem, it is an option that the holographic recording layer is supported on a substrate whose surface has an optical quality and which has rigidity similar to the conventional holographic recording medium. However, in the information retaining body whose periphery is rigidly formed so that the holographic recording layer is not deformed, conformity between a holographic memory and a support body is lost and a sense of usefulness is largely lost.
The display medium disclosed in Japanese Patent Application Laid-Open No. 2000-211257, in which the holographic image is printed, only utilizes the holographic image in a visual manner (decoration), and the invisible electronic information is not added to the display medium. Therefore, reproduction accuracy does not become a problem.